A key requirement for achieving high degrees of efficiency in solar cells is very effective suppression of surface recombination losses. For this purpose, the surface of solar cells should be passivated as effectively as possible, so that charge carrier pairs which are generated inside the solar cell by incident light and which diffuse to the surfaces of the solar cell substrate do not recombine at the solar cell surface, so that they would no longer be available to help improve the efficiency of the solar cell.
In laboratory solar cells, this problem is often solved by growing silicon dioxide at a high temperature (for example >900° C.). However, as a high-temperature process step of this type means considerable additional expenditure in solar cell processing, surface passivation of this type is at present generally not used in the industrial manufacture of solar cells.
A further difficulty of high-temperature oxidation is the sensitivity of more economical multicrystalline silicon in relation to high temperatures which can lead in this material to a considerable reduction in material quality, i.e. the charge carrier lifetime, and thus to losses in efficiency.
A low-temperature alternative is surface passivation using amorphous silicon nitride or silicon carbide which can be prepared at temperatures of 300-400° C. by means of plasma enhanced chemical vapour deposition (PECVD), for example. Surface passivation of this type is for example described in T. Lauinger et al.: “Record low surface recombination velocities on 1 □cm p-silicon using remote plasma silicon nitride passivation”, Appl. Phys. Lett. 68, 1232-1234 (1996); and in I. Martin et al.: “Surface passivation of p-type crystalline silicon by plasma enhanced chemical vapor deposited amorphous SiCx films”, Appl. Phys. Lett. 79, 2199-2201 (2001). However, the dielectric layers produced in this way can be used only to a limited degree for large-area, high-efficiency solar cells, as they can contain a high density of what are known as “pinholes”, i.e. small holes or pores in the layer, so that they may not have good insulating properties. In addition, their passivating effect is based largely on a very high positive charge density within the dielectric layers that can lead, during the passivation of the back of the solar cell if p-type silicon wafers are used, for example, to the formation of an inversion layer via which an additional leakage current of minority charge carriers can flow away from the base of the solar cell to the back contacts (what is known as a “parasitic shunt”). On highly boron-doped p+ silicon surfaces, silicon nitride can even lead, on account of the high positive charge density, to depassivation compared to an unpassivated p+ surface.
Very good passivations, both on p and on p+ surfaces, were achieved using amorphous silicon layers which can also be produced by means of plasma enhanced vapour deposition at very low coating temperatures (typically <250° C.), such as is described for example in S. Dauwe et al.: “Very low surface recombination velocities on p- and n-type silicon wafers passivated with hydrogenated amorphous silicon films”, Proc. 29th IEEE Photovoltaic Specialists Conf., New Orleans, USA (2002), p. 1246; and in P. Altermatt et al.: “The surface recombination velocity at boron-doped emitters: comparison between various passivation techniques”, Proceeding of the 21st European Photovoltaic Solar Energy Conference, Dresden (2006), p. 647.
However, the surface-passivating property of amorphous silicon layers of this type may be very susceptible to temperature treatments. In current-day industrial solar cell processes, the metal coating is in many cases carried out by means of screen printing technology, the last process step typically being a firing of the contacts in a continuous infrared furnace at temperatures between approx. 800° C. and 900° C. Although the solar cell is exposed to these high temperatures only for a few seconds, this firing step can lead to considerable degradation of the passivating effect of the amorphous silicon layers.
Good passivating results can also be achieved using aluminium oxide layers which are deposited by means of atomic layer deposition (ALD) at about 200° C., for example, and subsequently tempered at about 425° C. Nevertheless, in atomic layer deposition, only a single molecular layer of the material to be deposited is generally deposited on the substrate surface within each deposition cycle. As a deposition cycle typically lasts about 0.5 to 4 s, correspondingly low deposition rates are obtained. The deposition of aluminium oxide layers at a thickness which is suitable for use as an antireflection layer or as a back reflector therefore requires deposition durations which have in the past shown a use of such layers in industrially produced solar cells to be commercially disadvantageous.